Saving a post clears all categories

I use categories on my blog https://sm4good.com/. When I select a category and post the blog post, everything works as expected. The post appears under the correct category.

However, if I go back to the “edit” window – even if it has never been closed – all ticks have been removed and the post is now uncategorized. If I make a change to the post and publish it again, then it shows as uncategorized.

I have observed that the categories get cleared as soon as I save a post – this is the case with both drafts (Save) and published articles (Publish/Update).

I only noticed this behaviour today after upgrading to 3.9.

In summary, here is what happens:

– Categories are ticked manually
– Post is saved/published (with correct categories on website)
– Categories have been removed by system from post in “edit” view.
– Categories can be ticked again manually

The obvious workaround is to always tick the categories before saving/publishing but that is not really a solution. Has anyone else had the same problem?

[ No bumping please. ]